The invention concerns a catalyst system and procedure for selective alkylation of toluene.
There are two main methods to industrially produce alkylbenzene. One if Friedel-Crafts alkylation, which has the drawback that it tends to lead to polysubstitution (ring substitution) and, thereby, to difficult separation problems.
The other method uses base catalysts, such as Li, Na or K metals in the reaction between aromatic hydrocarbons and olefines. It is usual practice to use e.g. a K.sub.2 CO.sub.3 carrier. An efficient side chain alkylating catalyst is obtained when metallic sodium is dispersed on the surface of dry potassium carbonate. An alkali metal catalyst produces a smaller number of different isomers than a Friedel-Crafts catalyst. The drawback is the comparatively low selectivity of the alkali metal catalyst to aromatics and its tendency to produce various isomers of alkylbenzene, which are hard to separate. Aliphatic dimers are also formed, although these are easily separated from alkylbenzene by distillation.
The selectivity of an alkyl metal catalyst is lowered at the preparation stage by oxygen and water residues in the K.sub.2 CO.sub.3 carrier, whereby oxides and hydroxides are formed from part of the active metal. It is for this reason necessary to dry the carrier well at 120.degree.-150.degree. C. in vacuum for 10-20 hours and to prepare the catalyst in an inert atmosphere or in vacuum.
The x-ray diffraction spectrum run on unused catalyst reveals the presence of the following phases on the surface of the catalyst: Na.sub.2 O, K.sub.2 O, K.sub.2 CO.sub.3, K, and only a minor quantity of metallic Na and liquid, amorphous Na/K alloy, although the diffraction from the latter cannot be observed. Thus the catalyst has to be prepared in inert conditions in order to avoid oxidation. The alkali metal should be added in single doses as small as possible with the aid of a sodium press, or dispersed in an appropriate solvent; adequate dispersion is ensured in this way.
For improving the activity and selectivity of the Na/K.sub.2 CO.sub.3 alkylation catalyst, various organic promoters may be used, such as butadiene, anthracene, graphite, heterocyclic nitrogen compounds (methylpyridines) and "acetylenic hydrocarbons", and oxygenous hydrocarbons. The effect of organic promoters has been said to be based on formation of a complex between metallic potassium and the promoter, and this complex would have higher activity than the alkali metal alone. The stability of such a complex in the reaction conditions applied (&gt;150.degree. C.) is however unlikely. Various, better results have been achieved using inorganic promoters: for instance metallic copper, cobalt, titanium and ground steel have been tried. It is clearly observable that promoters of various characters exert an effect on alkylation and dimerization.
Catalysts like the catalyst of the invention are known in the art from other connections. As state of art is cited U.S. Pat. No. 3,260,679, in which a catalyst system is disclosed which contains metallic sodium on an aluminum oxide carrier. As promoter for the metallic sodium serves a transition metal compound, advantageously an oxide. In the preparation of this catalyst, the sodium is added onto the aluminium oxide carrier, the carrier being first heated in a dry atmosphere to about 200.degree.-600.degree. C. The sodium is added in fine powder form. The mixture is then agitated, whereby the sodium disperses on the surface of the aluminium oxide carrier. For transition metal oxide, in the reference iron oxide FeO.sub.3 is used. The impurities present in the catalyst have to be removed in order to attain sufficient life span of the catalyst. Use of these catalysts has been reported e.g. in conversion of 1-alkenes into 2-alkenes.